1. Field of the Invention
The present invention relates to a method for treating SO.sub.2, SO.sub.3 and a dust by combining a dry dust collector with a wet lime gypsum process exhaust gas desulfurizing apparatus for treating an exhaust gas containing SO.sub.2, SO.sub.3 and the dust.
2. Description of the Prior Art
At present, the most prevalent apparatus for desulfurizing an exhaust gas is an exhaust gas desulfurizing apparatus which makes use of a so-called wet lime process for carrying out the desulfurization of the exhaust gas by the use of CaCO.sub.3 or Ca(OH).sub.2 as an absorbent in order to recover SO.sub.2 in the exhaust gas in the forms of calcium sulfite and calcium sulfate (gypsum). Such a type of apparatus is described in detail in Japanese Patent Provisional Publication No. 63117/1982 and many other publications.
Now, reference will be made to the exhaust gas desulfurizing apparatus using the wet lime process which is widely employed on an industrial scale of late, in accordance with FIG. 2.
An exhaust gas 1 from, for example, a coal-fired boiler is treated in a denitrating device, an air heater and a dry dust collector, and the exhaust gas 1 containing the dust which has passed through the dry dust collector, SO.sub.3 and SO.sub.2 is then guided to an absorbing tower 2.
At the lower portion of the absorbing tower 2, a tank 3 for receiving a slurry in which calcium compounds are suspended is provided, and a stirrer 4 provided in the tank 3 stirs the slurry to prevent a solid from precipitating.
The slurry in which the calcium compound is suspended is delivered to the top of the tower 2 by means of a circulating pump 5, is then sprayed into the tower 5, flows down while brought into contact with the exhaust gas, and returns to the tank 3 again.
The exhaust gas from which SO.sub.2, has been removed by the contact with the slurry is discharged as a purified gas 7 through a mist eliminator 6.
On the other hand, the slurry of CaCO.sub.3 of Ca(OH).sub.2 is fed to the tank 3 via a line 8 in compliance with an amount of absorbed SO.sub.2, and the slurry containing calcium sulfite produced by an SO.sub.2 absorption of the absorbent is delivered to an oxidizing tower 10 via a line 9.
Into the oxidizing tower 10, air 12 is blown through an air bubble generator 11 disposed at the bottom portion of the tower 10, and sulfuric acid is blown thereinto through a line 13, whereby calcium sulfite is oxidized to a gypsum and unreacted CaCO.sub.3 or Ca(OH).sub.2 is converted into the gypsum.
The gypsum slurry going out from the oxidizing tower 10 is guided to a thickener 15 via a line 14, and the gypsum slurry which has been cocentrated therein is then delivered to a centrifugal separator 19 through a line 16, a tank 17 and a pump 18. In the separator 19, a gypsum 20 is formed, and a filtrate is delivered to a tank 21 and is further returned to the thickener 15 via a pump 22 and a line 23.
On the other hand, a supernatant liquid in the thickener 15 is introduced into a tank 25 through a line 24, and is then used partially for the sake of, for example, the regulation of the absorbent in the exhaust gas desulfurizing apparatus and the remaining supernatant liquid is drained simultaneously.
According to the process shown in FIG. 2, the dust (which has passed through the dry dust collector) contained in the exhaust gas 1 is caught in the absorbing solution, and is finally involved in the gypsum 20, which fact will deteriorate the quality of the gypsum. Thus, for the purpose of previously removing the dust in the exhaust gas therefrom, a cooling dust removing tower may be disposed on the upstream side of the absorbing tower. This system has widely been put into practice and is disclosed in, for example, Japanese Patent Publication Nos. 10,838/1977 and 12,026/1976.
The drawback of this conventional method resides in an additional necessity of disposing the cooling dust removing tower with the intention of preventing the dust from contaminating the by-product gypsum. Further, as be apparent from FIG. 2, the operative steps such as the oxidation, the precipitation and concentration of the gypsum and the recovery of the supernatant liquid are required separately and they are additionally complicated, which fact is uneconomical. The most troublesome point is that the drain treatment is essential, and this is the largest drawback of the wet exhaust gas treatment.
Since impurities such as the dust, HCl and HF present in the exhaust gas are caught in the absorbing solution and are discharged with the drainage through a different route than that of the gypsum, the drain treatment is inevitable from the viewpoint of the prevention of a secondary environmetal pollution due to the drain. Even in the case that any concrete description of the drain treatment is not made in disclosures regarding the wet exhaust gas treating technique, it should be understood that the technique requires such a drain treatment naturally.
Additionally, another large drawback of the conventional process is that SO.sub.3 in the exhaust gas cannot be removed therefrom, though SO.sub.2 can be absorbed and removed therefrom.
SO.sub.3 results from the oxidation of a part of a sulfur content at a combustion. In recent years, a denitrating device is often used, and by a secondary denitrating reaction, 0.5 to 4% of SO.sub.2 is oxidized, so that an amount of SO.sub.3 will increase. In consequence, the exhaust gas will contain 5 to 50 ppm of SO.sub.3, depending upon the sulfur content in a fuel.
When a temperature of the exhaust gas is lowered by a heat exchanger such as an air heater, SO.sub.3 is partially condensed to become a sulfuric acid mist, and the latter will adhere to the dust coming therewith. As a result, the dust will be acidic and will corrode the dry dust collector inconveniently at times.
The SO.sub.3 gas which has passed through the dry collector becomes the sulfuric acid mist by a temperature drop of the exhaust gas in the exhaust gas desulfurizing apparatus, but the thus formed sulfuric acid mist is composed of fine particles. Therefore, a part of the mist will be discharged from the desulfurizing apparatus through a chimney, so that it will be a cause of acid smut or white smoke or will corrode materials of the heat exchanger attached to the desulfurizing apparatus and a flue inconveniently.
Heretofore, a process is now put into practice which comprises introducing NH.sub.3 into an exhaust gas coming through an exit of a coal-fired air heater to form a reaction product (hereinafter referred to as the NH.sub.3 --SO.sub.3 reaction product) of SO.sub.3 and NH.sub.3 such as acid ammonium sulfate (NH4HS04) or ammonium sulfate [(NH.sub.4).sub.2 SO.sub.4 ], and collecting the thus produced NH.sub.3 --SO.sub.3 reaction product in a dust collector at a downstream position ("Mitsubishi Heavy Industries Technical Bulletin", Vol. 10, No. 5, 1973, p. 211 to 218).
However, if this method is utilized to remove SO.sub.3 in the exhaust gas containing a large amount of the dust from, for example, a coal-fired boiler, the following disadvantages will come out.
That is to say, both the NH.sub.3 --SO.sub.3 reaction product and the dust are caught in the dust collector, and the treatment of the dust containing a great deal of the NH.sub.3 --SO.sub.3 reaction product will be troublesome.
Heretofore, the dust discharged from the coal-fired boiler is effectively utilized as a material for a fly ash cement, or is thrown away for reclamation, but in the case of the former, an NH.sub.3 odor will be generated in mixing with water and a strength will deteriorate; in the case of the latter, the NH.sub.3 odor and a leak into underground water or the like will be problematical.
Furthermore, with regard to the exhaust gas from the heavy oil-fired boiler which is now brought into practical use, an amount of the dust is much smaller than in the exhaust gas from the coal-fired boiler, and thus the treatment of the dust containing the NH.sub.3 --SO.sub.3 reaction product is relatively easy. However, when the large amount of the dust is present as mentioned above, a great deal of the dust must be treated disadavantageously.
In the treatment of the exhaust gas from the coal-fired boiler in which NH.sub.3 cannot be used, the sulfuric acid mist will adhere to the collected dust in the dry collector as described above, so that the effective utilization of the dust might be obstructed. After all, with regard to the treatment for the exhaust gas containing the dust and SO.sub.3 in addition to SO.sub.2, any rational method has not been established yet.
The conventional wet lime/gypsum process desulfurizing method has at least the following drawbacks:
(1) The by-product gypsum is contaminated by the dust.
(2) In order to recover the dust-free by-product gypsum, the cooling dust removing tower has to be additionally disposed.
(3) In the course of from the absorption of SO.sub.2 to its recovery as the by-product gypsum, it is necessary to separately arrange the operative steps of the oxidation, the precipitation and concentration of the gypsum, and the recovery of a supernatant liquid.
(4) The treatment of the drain containing impurities coming from the exhaust gas and the like is essential.
(5) SO.sub.3 in the exhaust gas is converted into the sulfuric acid mist consisting of the fine particles by the drop in a gas temperature in the desulfurizing apparatus, but the thus formed mist cannot be removed. As a result, the mist will be a cause of acid smut or white smoke and will corrode materials inconveniently. The sulfuric acid mist will also adhere to the dust collected in the dry dust collector disposed on the upstream side of the wet exhaust gas desulfurizing device, and the effective utilization of the dust will be obstructed.